This invention relates to zopolrestat monohydrate, polymorphs thereof and to processes for preparing the polymorphs. This invention further relates to processes for using the polymorphs to prepare anhydrous zopolrestat, and to methods of using the polymorphs and to compositions comprising the polymorph to treat, prevent or reverse complications arising from diabetes mellitus. Zopolrestat, also known as 3,4-dihydro-4-oxo-3-((5-trifluoromethyl)-2-benzothiazolyl)methyl-1-phthala zineacetic acid, is the compound of formula I: ##STR1##
Zopolrestat is disclosed in U.S. Pat. No. 4,939,140 together with its utility in inhibition of aldose reductase, more specifically having utility in the treatment of complications arising from diabetes mellitus such as diabetic cataracts, retinopathy and neuropathy. Subsequently, zopolrestat has been disclosed as having utility in lowering blood lipid levels (U.S. Pat. No. 5,391,551) and in lowering blood uric acid levels (U.S. Pat. No. 5,064,830).